Ultraviolet (UV) fluorescence of native proteins and neurotransmitters can provide a direct window on the mechanisms of cellular function once the difficulties of photodamage, scattering and indiscriminate background accompanying UV excitation are surmounted. As a solution, we have explored the possibility of exciting UV molecular fluorescence by simultaneous absorption of three infrared photons. A scanning microscopy technique using three-photon excited fluorescence has been developed, allowing, for the first time, direct three-dimensionally resolved imaging and quantitative detertmination of protein and neurotransmitter distributions in living cells.